Make shared -DSMALL libc buildable without breaking libhack
We used -DSMALL to exclude code from libc in order to build
libhack. Introduce -DLIBHACK to do this without so that
-DSMALL does not remove code necessary for building a shared libc

PR/45989: Martin Husemann: lint invocation does include -w only on i386
- turn lint -w for all the platforms after fixing the lint warnings.
- add _DIAGASSERTS() for casts that would assign values to types that
would not fit.
- change types, add casts
- change into ansii prototypes
- turn on _DIAGNOSTIC for libc (during current, to be eliminated for release
builds)
approved by core@

Overhaul how `build.sh tools' are used:
* Rename "config.h" to "nbtool_config.h" and
HAVE_CONFIG_H to HAVE_NBTOOL_CONFIG_H.
This makes in more obvious in the source when we're using
tools/compat/config.h versus "standard autoconf" config.h
* Consistently move the inclusion of nbtool_config.h to before
<sys/cdefs.h> so that the former can provide __RCSID() (et al),
and there's no need to protect those macros any more.
These changes should make it easier to "tool-ify" a program by adding:
#if HAVE_NBTOOL_CONFIG_H
#include "nbtool_config.h"
#endif
to the top of the source files (for the general case).

* use _DIAGASSERT() to check pointer arguments against NULL and file
descriptors against -1 (as appropriate).
* add actual checks which to detect stuff that would trigger_DIAGASSERT(),
and attempt to return a sane error condition.
* knf some code
* remove some `register' decls.
the first two items result in the addition of code similar to the
following in various functions:
_DIAGASSERT(path != NULL)
#ifdef _DIAGNOSTIC
if (path == NULL) {
errno = EFAULT;
return (-1);
}
#endif

PR 5170: don't fclose twice, or call ferror after fclose.
required removing fcloses and/or moving them, nulling the pointers
like they should have been, and adding a terminating null byte and break
in one case (perhaps we never ran into those problems before).
cgetclose() already does a fclose and null of the pointer, so don't
duplicate it when we don't need to.

If port provides __weak_alias(), provide an Standard C and POSIX pure
identifier namespace by renaming non standard functions and variables
such that they have a leading underscore. The library will use those
names internally. Weak aliases are used to provide the original names
to the API.
This is only the first part of this change. It is most of the functions
which are implemented in C for all NetBSD ports. Subsequent changes are
to add the same support to the remaining C files, to assembly files, and
to the automagically generated assembly source used for system calls.
When all of the above is done, ports with weak alias support should add
a definition for __weak_alias to <sys/cdefs.h>.